1. Field of the Invention
The invention relates to a distance measuring device according to the preamble of Claim 1 and to a distance measuring method according to the preamble of Claim 9.
2. Description of the Background Art
In electro-optical distance measuring devices (EDM), an optical signal is emitted from the apparatus in the direction of the target object whose distance it is necessary to determine, for example as optical radiation in the form of laser light. If visible light is used in this case, then the point aimed at for measurement on the target object can be visually discerned. By contrast, if non-visible wavelengths, e.g. in the infrared range, are used or if the target object is further away, then aiming at the point to be measured can be carried out by means of an optical device, for example by means of a crosshair in an observation device.
The surface of the target object reflects at least part of the optical signal, usually in the form of a diffuse reflection. The reflected optical radiation is converted into an electrical reception signal by a photosensitive detector element in the apparatus. With knowledge of the propagation velocity of the optical signal and with the aid of the propagation time determined between emission and reception of the signal (that is to say that propagation time which is required by the light for covering the distance from the apparatus to the target object and back), it is possible to determine the distance between apparatus and target object. In this case, optical components for beam shaping, deflection, filtering, etc.—such as, for instance, lenses, wavelength filters, mirrors, etc.—are usually situated in the optical transmission and/or reception path. In this case, the emission and reception can be effected coaxially or by means of two adjacent optical units. Distance measuring devices of this type can be designed as independent apparatuses, but can also be integrated into other apparatuses, for example into surveying apparatuses such as theodolites or tachymeters, or into observation apparatuses such as telescopes, monoculars, binoculars, night vision apparatuses, etc.
In order to compensate for influences which might corrupt the measurement results (for example temperature influences, component tolerances, drifting of electronic components, etc.), part of the emitted optical signal can be guided as a reference signal via a reference path of known length from the light source to a light-sensitive receiving element. In this case, the reference path can be fixedly incorporated in the apparatus or be designed for example as an optical deflection element that can be pivoted in or plugged on. The reception signal resulting from said reference signal can be received by the photosensitive element which is also used for measurement or by a further photosensitive element provided especially for the reference signal. The resulting electrical reference signal can be used for referencing and/or calibrating the measured values determined.
In order to obtain a correspondingly high accuracy of the distance measurement, on account of the high propagation velocity of optical radiation, the requirements made of the temporal resolution capability in electro-optical distance measuring devices (EDM) are extremely high. By way of example, for a distance resolution of 1 m, a time resolution having an accuracy of approximately 6.6 nanoseconds is required.
The measurement requires sufficiently high signal intensities for the returning reception signal that are able to be detected by the receiver. The signal power which can be emitted for the transmission signal from the optoelectronic EDM under consideration here is limited by physical and regulatory limits, however. In many cases, pulsed operation is therefore used. The emitted optical signal thus has its intensity amplitude modulated in a pulse-like manner. Temporally short pulses having a high peak power are emitted, followed by pauses in which no emission of light takes place. Hence, the reflected portion of the pulses has sufficiently high intensity to allow said pulses to be evaluated from the background disturbances and noise, particularly also when background light (sunlight, artificial lighting, etc.) is present. The number of pulses per packet can be varied, depending on the evaluation concept and the measurement situation, from single pulses through to a quasi-continuous pulse train.
In order to determine the propagation time of the signal, firstly the so-called time-of-flight (TOF) method is known, which determines the time between the emission and reception of a light pulse, the time measurement being effected with the aid of the edge, the peak value or some other characteristic of the pulse shape. In this case, pulse shape should be understood to mean a temporal light intensity profile of the reception signal, specifically of the received light pulse—detected by the photosensitive element. The point in time of transmission can be determined either with the aid of an electrical pulse for initiating the emission, with the aid of the actuating signal applied to the transmitter, or with the aid of a reference signal mentioned above.
Secondly, the so-called phase measuring principle is known, which determines the signal propagation time by comparison of the phase angle of the amplitude modulation of the transmitted and received signals. In this case, however, the measurement result in the case of one transmission frequency has ambiguities in units of the transmission frequency period duration, thus necessitating further measures for resolving these ambiguities. By way of example, WO 2006/063740 discloses measurement with a plurality of signal frequencies which result in different unambiguity ranges, as a result of which incorrect solutions can be precluded. WO 2007/022927 is also concerned with unambiguities in phase measurement.
When the principles of distance measuring methods are considered mathematically, it is possible to see a dependency for the attainable distance measuring accuracy on the emitted light pulses. As explained in the book “Electro-Optical Instrumentation—Sensing and Measuring with Lasers” by Silvano Donati, in chapter 3.2.3, for example, it is possible to see that the measuring accuracy is dependent on the reception signal energy (that is to say on the number of photons received). The embodiment of distance determination using a threshold value for the reception signal edge which is described in this section of the book can be regarded as exemplary in this case and merely as a good example. The underlying principles and the results thereof can similarly also be applied to other evaluation methods for ascertaining distance by determining the propagation time of light pulses.
Specifically, equation 3.16,σ1=τ·(factor)shows that the period of the pulse τ has an influence during determination of the attainable measurement error σt, particularly an essentially linear influence, scaled using a factor which combines a wide variety of other influencing variables. This fundamental result also continues to be valid when the algorithm used for evaluating the distance is varied, specifically since the physical principles of measurement remain the same. The result is also largely independent of the signal shape received.
Since the measurement inaccuracies therefore improve linearly in a first approximation as the pulses used become shorter in time, a higher level of measurement accuracy can be achieved with shorter pulses.
The intensity of the received portion, reflected from the target object, of the emitted light is dependent on various factors and can therefore also vary greatly. In this case, an influencing variable that can be mentioned is the distance from the target object, with which the expansion of the emitted beam of measurement light increases, and also the signal attenuation as a result of atmospheric disturbances such as mist, fog, heat haze or dust increases on account of the longer path. In distance measuring devices, large working ranges are often required in this case, for example from the dm range to the km range. In observation apparatuses having distance measuring devices (for example night vision apparatuses, telescopic sights, telescopes, binoculars, etc.), there are often required ranges of from a few meters to many kilometers, for example from 5 m to 20 km or 30 km, and this is with a measurement accuracy of a few meters or even less, for example of from ±1 m to ±5 m or less.
A significant factor in this case is the signal-to-noise ratio (SNR) of the information used for determining the distance. The SNR attained is, inter alia, also one of the main criteria for the maximum measurement distance that can be attained.
If the received signal is too weak, then it can no longer be identified unambiguously from the background noise or the ambient light. On account of the poor signal-to-noise ratio, only an inaccurate or in the worst case no distance measurement at all is possible. In order to be able to perform a measurement, the SNR needs to exceed a certain minimum level at which the signal can be discerned from noise and hence it is actually possible to perform a measurement. Particularly by virtue of statistical methods, averaging operations, etc., this is entirely possible even with a low effective SNR for a single received pulse. In the case of a repeated train of short pulses and summation or averaging of the resultant measurement signals, a higher SNR can be attained. The statistical averaging performed in this case can prompt an improvement in the SNR by averaging out nondeterministic error influences, such as noise. Numerical averaging of a plurality of measurements following the A/D conversion can also prompt the significant bits of the digital reception signal representation to be expanded. By way of example, correct-phase accumulation of a plurality of reception pulses may allow measurement even at SNR values for the single pulses below 1. With N reception pulses, this results in an improvement in the signal-to-noise ratio (SNR) by approximately a factor of √N, this effect not being able to be extended to any number N, but rather the improvement becoming saturated from a certain number of pulses onwards.
An SNR above a minimum threshold is a basic requirement to be able to perform a measurement in the first place. A further increase beyond this minimum threshold allows greater measurement accuracy, greater reliability of the measurement or shorter measuring periods (for example because fewer signals need to be accumulated in order to obtain an evaluatable signal). However, the aforementioned improvements also decline from a certain SNR level onward and then have only a marginal effect on the measurement result, for example because other limiting effects prevail.
In this case, the noise term N is formed primarily by the signal transmission path, that is to say by the measurement distance over which the light travels, by external interfering influences, such as ambient light and (usually to a substantial degree) by the noise in the electronic receiving circuit. In this case, N is also dependent on the ambient conditions such as temperature, etc., which need to be accepted as pre-existing. Particularly in apparatuses having a distance measuring device for field use, temperature ranges of from −20 to +60° C., for example, or greater are often required. Although the receiving circuit is designed to have as little noise as possible, this cannot be avoided totally.
The signal term S also has limits set. Besides the reflectivity of the target for the measurement radiation, which reflectivity cannot be influenced without using specially prepared targets, the received intensity of the reflected signal also decreases as the measurement distance increases, for example. Although it is possible to subsequently raise the signal level through amplification, this also always amplifies the noise (which occurs before and in the amplifier). The amplifier used for this is also an additional noise source in the system.
An obvious approach to increasing the signal strength of the reception signal is to increase the signal strength of the transmission signal. If it is assumed that the attenuation of the signal path with respect to the signal strength is linear, this prompts a linear increase in the SNR. Hence, by way of example, doubling the transmission intensity could also (at least approximately) achieve doubling of the SNR of the reception signal. The maximum possible transmission intensity has limits set, however, specifically by physical limits, for example the light source used, the actuating circuit, the available transmission power or supply power, thermal limits, etc., specifically with regard to constraints such as small physical size, battery operation, component costs, etc., which pre-exist specifically in the case of a mobile distance measuring device.
If the received optical signal has too great an amplitude, on the other hand, then this can result in saturation in the receiver, e.g. for the receiving element, an amplifier stage or an A/D converter. Such saturation makes accurate distance measurement difficult or impossible. The signal dynamic range of the receiving circuit is thus limited on the basis of hardware, which also limits the possible range of the S term. By setting the pulse amplitudes, it is possible to adopt different target object reflectivities and signal attenuations along the measurement path, for example.
The document WO 97/22179 describes a circuit arrangement for a pulse output stage which can be used as a driver stage for feeding a light source and for emitting light pulses. This circuit can be used to vary the amplitude of the emitted pulses and hence the energy emitted per pulse. The duration of the pulses produced is fixedly prescribed by the circuit design, however. DE 23 31 084, DE 199 43 127 and U.S. Pat. No. 3,628,048 also show similar circuit concepts. The durations of the pulses in this case change at most as a by-product of the amplitude change, for example as a result of changing signal edge gradients or signal edge levels.
The range of variation of the transmission pulse amplitude has limits set even with such a pulse output stage, however. Firstly, the saturation of the receiver, explained previously, needs to be avoided. If, by way of example, a constant radiation portion is coupled from the transmitter directly onto the receiver as a reference signal via a reference path, the admissible amplitude dynamics of the transmitter need to match those of the reference receiver. The radiation source of the actuating circuit therefor may also be a limiting factor with a maximum possible pulse amplitude value.
A further known starting point for improving the SNR is to filter out the undesirable noise component from the measurement signal. Specifically, this involves matching the bandwidth (BW) of the received signal to that of the useful signal, that is to say filtering out high frequency or low frequency noise and also other interference signals using digital and/or analogue filters, for example. In this case, the SNR can be improved by a factor of √BW, for example. In this case too, practical implementability has limits set, since steep filters of high quality and high linearity in amplitude and phase can be implemented only with difficulty in practice. By way of example, it is necessary to avoid aliasing effects during digitization by adhering to the Nyquist criterion sufficiently well. Specifically analogue filters having appropriate characteristic values are a challenge in terms of circuitry. On the other hand, alternative digital filtering requires the use of correspondingly higher sampling rates and is therefore likewise complex and costly in terms of circuitry.
In this case, excessive limiting of the bandwidth has a negative effect on measurement accuracy, since trimming the high frequency components also entails a loss of signal information. This is also consistent with the assessment explained further above that higher measurement accuracy can be achieved with short—and hence more wideband—pulses. The bandwidth of the receiver should therefore be attuned (at least approximately) to the bandwidth of the transmitted pulse signal.
The known solutions from the prior art in their known form are therefore a compromised solution as regards the signals used for distance measurement. Particularly in the case of distance measuring devices in observation apparatuses, which are not intended for highly accurate geodetic land surveying, for example with measurement accuracies approximately in a range of one meter and with measurement ranges of up to 5, 10, 20 or 30 km or more and which generally require no special target marks for measurement, the apparatus design in the prior art designs and stipulates the signal used such that it is possible to measure distance over the entire specified measurement range (or the range specified on the basis of the possibilities of the signal used).
It is an object of the present invention to improve an optoelectronic distance measuring device.
In this case, it is also an object to expand the scope of distance measurement for an EDM, that is to say that distance range in which distance measurement is possible, specifically without losing measurement accuracy at short range.
In particular, it is an object to improve the relative distance measuring accuracy for the distance over the measurement range, specifically in each case at the upper and lower ends of the capturable measurement range.
It is a further object of the present invention to improve and simplify the distance measuring device by simplifying matching parameters of the distance measurement to the circumstances of the respective measurement task.
It is also a specific object to provide user-selectable measurement modes which can be used to meet the respective requirements of a measurement task. In this case, the user can be provided with the freedom of choice between greater maximum distance range—but with a less accurate distance measurement value, or a shorter maximum distance range—but with more precise measurement of the distance value.
As a specific task, the aim is to provide a transmitting unit for a distance measuring device, particularly a semiconductor laser driver stage as a pulse output stage, which can be used to influence the emitted pulses not only in terms of their amplitude value but also in terms of their pulse duration.
According to the invention, short pulses, which are evaluated using a large bandwidth in the receiving circuit used, are applied for correspondingly high signal strengths (that is to say specifically for short distances, highly reflective targets and/or good visibility). These short pulses allow—when evaluated at high frequency—accurate distance measurement. In this context, a large bandwidth can be understood to mean particularly a receiving circuit bandwidth that is as large as possible on the basis of hardware and that is prescribed as a design criterion with the circuit layout.
If the received signal intensity in comparison with noise is insufficient to perform a measurement successfully (specifically in the required time), the invention involves the emission of pulses of relatively long pulse duration which can also be evaluated with a lower bandwidth as appropriate. This allows more narrowband filtering and better noise rejection and hence an improvement in the SNR.
Since the hardware receiving circuit is designed for the higher bandwidth of the short pulses anyway, it is particularly possible to apply digital filtering in software or hardware in this case. Although the alternative of bandwidth matching for an analogue filter is likewise possible, switching analogue filter coefficients can be complex, can increase the complexity and component outlay for the circuit and can generally reduce circuit performance.